Johnson, 55, was admitted to St Thomas’ Hospital in central London late on Sunday after suffering persistent coronavirus symptoms, including a high temperature, for more than 10 days.
His condition rapidly deteriorated over the next 24 hours, and he was moved to an intensive care unit – where the most serious cases are treated.
Although he had received oxygen, his office said he was still conscious.
Downing Street said the move to intensive care was “a precaution should he require ventilation to aid his recovery”.
Britain has no formal succession plan should a prime minister become incapacitated.
“The PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary,” Downing Street said.
Just hours earlier, Johnson’s office said he was in good spirits and Raab had told a news conference that the prime minister was still running the government.
“The government’s business will continue,” a sombre Raab, 46, told reporters, saying Johnson was in the safe hands of a brilliant medical team.
“The focus of the government will continue to be on making sure that the prime minister’s direction, all the plans for making sure that we can defeat coronavirus and can pull the country through this challenge, will be taken forward.”
Johnson tested positive for the virus on March 26, the first leader of a major power to announce that he had tested positive for COVID-19.
After 10 days of isolation in an apartment at Downing Street, he was still fighting the virus on Sunday evening with a high temperature and persistent cough, prompting his admission to hospital. He was last seen in a video message posted on Twitter on Friday when he looked weary.
“There is no doubt this turn of events means Boris Johnson is extremely sick,” said Derek Hill, a professor of medical imaging at University College London (UCL).
Buckingham Palace said Queen Elizabeth had been kept informed by Downing Street and US President Donald Trump said all Americans were praying for his recovery.
“He’s been really something very special – strong, resolute, doesn’t quit, doesn’t give up,” Trump told a news briefing, adding he had asked two pharmaceutical firms developing potential COVID-19 therapies to get in touch with the British government to offer their services.
“We’ve contacted all of Boris’ doctors and we’ll see what is going to take place,” he said.
“But they are ready to go.”
Johnson and his health minister tested positive last month and chief medical adviser Chris Whitty self-isolated. Johnson’s pregnant 32-year-old fiancée, Carrie Symonds, also had symptoms but said on Saturday she was feeling better.
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